Chimneys take more weather punishment than almost any other part of a roof — they stand proud of the roofline, catching wind and rain from every direction — and the mortar holding the brickwork together is usually the first thing to show it.
What to look for from the ground
Crumbling, cracked, or visibly missing mortar between the bricks is the clearest sign, and it's often visible with binoculars from the garden without needing to get up there. White, powdery staining on the brickwork (called efflorescence) is another indicator — it's caused by moisture moving through the brick and depositing salts on the surface as it dries, which suggests water is getting further into the structure than it should.
Any visible movement or leaning in the stack itself is more serious and worth getting looked at promptly rather than waiting.
The connection to leaks inside the house
A surprising number of "roof leaks" that show up as damp patches on a chimney breast wall inside the house aren't actually roof leaks at all — they're chimney mortar that's failed, letting water track down inside the stack and into the room below. It's worth mentioning this specifically if you've got a damp patch near a chimney breast, since it changes what actually needs fixing.
Repointing vs. capping
Repointing fixes failing mortar and restores the stack's structural integrity. If the chimney is no longer in use, it's also worth having it capped — an uncapped, unused flue is an open route for rain and nesting birds, and a proper cap keeps both out while still letting the flue breathe. See our chimney repointing & capping service for how we approach both.